(1) Field of the Invention
The present invention relates to lubricants for gas compressors.
(2) Description of Related Art
Mixtures of low viscosity lubricant base stock with high viscosity lubricant base stocks are often referred to in the art as “dumbbell” blending. The result is a blended base stock with a viscosity intermediate to the two base stocks. In some cases the result also includes an improvement in the viscosity index, also called VI, as described in ASTM D-2270, (ASTM International provides standards worldwide). The same results can be obtained if more than two base stocks are blended. In general, if a low viscosity index base stock is blended with a high viscosity index base stock, then the result obtained is normally a blend with a viscosity index higher than the low viscosity index base stock. If the low viscosity base stock and the high viscosity base stock each have the same viscosity index, the resulting viscosity index for the blend in general is equal to or greater than that viscosity index.
It is well known that blends of certain base stock types can result in a viscosity index greater than either of the original two base stock. Polyalphaolefins or PAOs are a type of synthetic hydrocarbon. They are generally classified by their viscosity in cSt at 100° C. as determined by ASTM D-445. For example a PAO 4 would have approximately a viscosity of 4 cSt at 10° C. and a PAO 40 would have approximately a viscosity of 40 cSt at 10° C. In one study, the viscosity index of a PAO 6 was a measured at 119 and the viscosity index of a PAO 40 was measured at 142. A mixture of approximately equal portions of the two PAO base stocks resulted in a viscosity index of 149.
Compressors used to compress gasses that can be soluble in the compressor fluid require the selection of a sealing and lubricating fluid that will result in a viscosity sufficient to seal the compression area and to provide the required lubrication to bearings, gears and other mechanical parts. Lubricants that are to be used in reciprocating gas compressors must provide lubrication for the crankshaft and other portions of the drive train and transmission parts of the compressor, and they must provide lubrication for the compression chamber. The lubrication of the drive train and transmission parts of the compressor requires an extremely stable material that retains its viscosity and lubricating properties under various extreme conditions. A second function is to help seal piston rings.
There are several types of reciprocating compressors. Lubrication points include cylinders, valves, pistons, piston rings, crankshafts, connecting rods, main and crank pin bearings, and other associated parts. Double-acting machines use crossheads and crosshead guides with connecting pins to join the crosshead to connecting rods. Most single-acting compressors use connecting rods attached directly to the pistons with wrist pins or piston pins. “Oil-free” machines do not require lubrication in the compression area.
Cylinder lubrication is applied to cylinder parts, pistons, rings, valves, and rod packings. Crankcase parts include main and crank pin, crosshead (or wrist pin) bearings, and crossheads and crosshead guides. Additionally, the lubricant may help the efficiency of seals and minimize their wear. The crankcase on a reciprocating compressor can either be open to the cylinder (as in many vertical, V-type, and radial compressors) or sealed from the cylinder by a bulkhead and exposed to air (horizontal compressors). Bearing and other crankcase components require relatively large amounts of lubricant, which is usually supplied from the crankcase. Supply methods include “splash” (utilizing dippers in the oil), “flooded” (devices to lift the oil such as disks, screws, grooves, or oil ring gears), and “forced-feed” systems.
Cylinder lubricant may be “splashed” from the crankcase or “force-fed” from either the crankcase or a separate reservoir. Ideally, the amount of lubricant used is the minimum needed to provide a strong lubricant film (to minimize wear and friction, to seal piston rings, valves, and rod packings), remove heat, and prevent corrosion.
A machine with the crankcase open to the cylinder can experience gas leaks past the oil control rings and into the oil. In systems in which the crankcase is not open to the cylinder (i.e., the oil is fed to the cylinder walls and piston rod packings with a force-feed lubrication system) essentially all the oil fed to the cylinder eventually leaves the compressor with the gas.
Screw compressors, or rotary screw compressors are constant volume devices having a built-in compression ratio. Compression in the single-stage, double-helical type occurs through the meshing of two rotors in a one-piece, dual-bore cylinder. The cylinder has gas inlet passages, oil injection, a compression area, and discharge ports. Rotors are designated as male, with helical lobes, and female, with corresponding helical grooves. In oil-flooded machines, the lubricant is injected into the compression area, affording sealing via an oil film between the intermeshing screws, and removal of the heat of compression. Oil separators are used to remove the oil from the discharge gas. “Dry screw” machines utilize timing gears to position the screws so that no internal lubrication is required.
Liquid-injected, single-screw compressors are constant volume, variable pressure machines. Compression results from the intermeshing of a single screw with one or two gate rotors. The screw and casing combine to act as a cylinder. The gate rotor or rotors act as a piston. The screw also provides the action of a rotary valve, the screw and gate act as a suction valve, and the screw and casing (port) serve as a discharge valve. There is a relatively low amount of friction between the screw and gate, as nearly all the compression is supplied by the screw. Bearings can be lubricated by grease or fluid, depending on design.
There are two common types of vane compressor: fixed and rotating. Both types provide positive-displacement, non-reversing compression. The fixed-vane type uses a ring or roller, which rotates around an eccentric shaft. A single vane is mounted in a non-rotating cylinder housing. The rotating-vane compressor has a rotor concentric with the shaft and off-center with respect to the cylinder housing. The rotor is equipped with radially sliding vanes, which are forced against the cylinder walls by centrifugal force. Gas is trapped between the vanes and wall, where a reduction in volume serves to compress it.
The lubricant in rotary vane compressors helps to provide a seal between the sliding vanes and the cylinder (or ring) wall. Larger systems may use oil pumps. Adequate lubrication should be provided to the vanes, vane slots, bearings, and seal faces. The oil to the cylinders may be supplied from the bearing lubricant discharge. The lubricant also prevents gas leakage in rotating shaft seals.
The basic compression unit in a scroll compressor is a set of two scrolls, one fixed and the other moving in a controlled orbit around a fixed point. Areas of lubrication include a short throw crank mechanism, bearings, and the scroll tip. Sealing is achieved through very accurate machining, proper balancing of pressures between scrolls, linkage mechanisms, and sometimes a sealing element at the tip of the involute.
The compressor working fluid in compressor applications which compress gasses which are soluble consists of the base stocks, additives, dissolved gasses and any condensate from those dissolved gasses or condensates of materials that are carried in with the feed gas. This working fluid is used to lubricate compressor mechanical parts, seal compression areas, seal other areas such as pumps and seal housings, and in some cases provide a method of removing the heat of compression. The working fluid in these compression systems influences the operation and efficiency of the entire system. Some of this fluid is carried out of the compressor and into the system. The interactions of the working fluid will impact the return of this fluid to the compressor. The solubility of the components from the gas being compressed in the working fluid, either as dissolved gas or gas condensates, impacts compressor performance. Dissolved gas in the working fluid's base stock(s) reduces the viscosity of the working fluid. Excessive dissolved gas can lead to wear and/or inefficient compression. The effect of the solubility of the gas on the ability of the working fluid to lubricate is a major concern. Excessive dilution may cause a reduction in viscosity resulting in a loss of the working fluid's film thickness. The lubricant can be washed off wear surfaces by liquid components of the gas. Additional problems can occur when there is a reduction of pressure, followed by degassing (foaming, cavitation, loss of lubricant film). A high degree of solubility of base stock components with the gas can result in loss of the base stock through absorption into the gas phase. This can result in high working fluid feed rates and a source of contamination to the gas leaving the compression system.
The working fluid has more effect on the performance of a screw compressor than is the case with a reciprocating compressor, primarily because of differences in the design of the two oil systems. The screw compressor injects the working fluid at discharge pressure into the compression chamber. This oil is then removed from the compressed gas by the use of an oil separator and sump situated on the high pressure side of the system. The screw compressor benefits from a working fluid derived from base stocks which exhibit limited solubility and a higher viscosity with the gas at discharge conditions (at the oil separator) to achieve high performance. Limited solubility will reduce or eliminate bypass of the gas being compressed from discharge to suction or to a lower situated thread. External bypass due to the gas circulating with the base stock is also reduced, leading to both high volumetric efficiency and low torque.
The lubrication and sealing of gas compressors encompasses many different types of gas, which can be categorized as inert, soluble, or reactive. The type of gas (hydrocarbon, carbon dioxide, etc.), the functionality of the base stocks used, and the performance of the compressor must all be considered. There are three distinct areas of concern in gas applications: solubility, reactivity, and the effect of the lubricant as a contaminant in the compressed gas. The first two affect the compressor performance and the latter the gas application. Under certain pressures and temperatures even so called “insoluble gasses” can have enough solubility to influence the viscosity or lubrication properties on the working fluid and so are included in the subject of this invention.
Reactions of the gas with the lubricant can result in premature failure of the compressor or, in more severe cases, fires and explosions. Base stocks or additives may react with or inhibit catalysts, cause mechanical problems in the application (valves etc.), or plug areas of gas flow.
The compression of soluble gasses often involves the selection of synthetic working fluids due to their unique viscosity-temperature relationships or for their resistance to dilution by the gas or its condensates. The major difference of compressor working fluids used in soluble gas applications from lubricants or fluids used with non-soluble gasses or in general those lubricants used in industrial lubrication is that the compressor working fluid in soluble gas applications consists of the base stocks used and the gas or gas condensates that may dissolve in the compressor working fluid. These dissolved gasses or gas condensates have an effect on the viscosity and viscosity temperature relationship of the compressor working fluid. Additives can also be used in the formulation of compressor working fluids used in the compression of soluble gasses. Certain types of base stocks have been found to reduce or eliminate the effects of these dissolved materials, but do not always provide the best overall choice for a working fluid. Additional factors such as potential reactions with catalysts used in process systems, low temperature fluidity, viscosity temperature characteristics, stability, wear characteristics or even cost can eliminate the potential use of certain types of base stocks.
Hydrocarbon gases are types of soluble gases that are encountered in the collection and transmission of natural gas, in vapor recovery, in landfill gas compression, in the petroleum and chemical industries. These “gasses” often are mixtures of hydrocarbon components, hydrogen, carbon monoxide, carbon dioxide, hydrogen sulfide and can include other chemical components used in the process. Certain types of base stocks are often selected for their resistance to dilution by hydrocarbons. These are often oxygen containing materials such as polyglycols or polyalkylene glycols and their derivatives. Other types of synthetic base stocks such as esters, ethers, silicones, and halogen containing fluids have also been used to reduce dilution.
Compressed natural gas and other hydrocarbon gases are used to fuel gas turbines. The compressor supplies the gas at the flow rate and pressure needed for continuous operation of the turbine. Petroleum-based lubricants carried in the gas may produce carbonaceous deposits in the gas inlet nozzles of the turbine, restricting flow and causing flameout.
Hydrocarbon gases are often the feedstock for a chemical process. Examples include ethylene and propylene used in the manufacture of polyethylene and polypropylene. Synthetic oils are often used for these applications because they do not react with or inhibit the catalysts. Additionally, synthetic lubricants can be used in the production and handling of ethylene and propylene as a result of the same concerns.
In reciprocating compressor applications, for pressures below 1000 psig, ISO Viscosity Grade 100-150 mineral oils can be used. Problems occur when the gas is wet or at increased pressures. The addition of fatty oils is common. These additives, which are difficult to pump at low temperatures, can cause damage to discharge valves, accumulate in aftercoolers and piping, and emulsify with water.
High pressure reciprocating compressors (5000 psig) are used to re-inject natural gas into crude. Four basic problems have been identified with this process: (1) loss of lubricant viscosity, (2) increased cylinder wear rate because lubricant was washed off surfaces by liquid components in the gas, (3) loss of lubricant to the high pressure gas stream, which results in feed rates to rod packings of 10 times normal rates, or up to a barrel of lubricant per day per compressor, and (4) reaction of additives with well-bore fluids, leading to permanent impairment to the well and reduced gas injection rates.
Viscosity is the most critical lubricant requirement that must be met in the hydrocarbon gas rotary screw compressors. Viscosity can be lowered as the lubricant is diluted by the hydrocarbon gas in the compressor and oil separator. The final level of dilution is determined by the temperature and pressure in the separator, which is located on the discharge side of the compressor. Synthetic lubricants offer the advantage of very high viscosity and, in some cases such as with polyalkylene glycols, are resistant to dilution (lower solubility with hydrocarbon gases).
Even though synthetic lubricants have improved the lubrication and efficiency of gas compressors, there is a need for improvement.